Rub tolerant fan case

ABSTRACT

A case apparatus for a gas turbine engine includes an annular case having an interior surface with annular recess formed therein; and an annular bumper disposed in the recess, the bumper comprising a frangible material and having a low-friction contact surface, wherein the bumper is configured to permit elastic radial deflection in response to applied forces below a predetermined threshold.

BACKGROUND OF THE INVENTION

This invention relates generally to gas turbine engines and more particularly to a containment structure for a fan of a gas turbine engine.

A turbofan engine typically includes a fan, a booster, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine in serial axial flow relationship about a longitudinal centerline axis of the engine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and booster via a second rotor shaft. The fan includes an annular disk and a plurality of radially extending blades mounted to the disk, wherein the disk and the blades are rotatable about the longitudinal centerline of the engine. Such fans are surrounded by a fan case which is specifically designed to be capable of containing a fan blade in the event that the fan blade is released from its disk during operation. This prevents or minimizes the structural damage to the engine and aircraft should one or more fan blades be released from the disk due to a catastrophic failure of one or more blades, ingestion of debris, or other cause.

The fan case also serves as the outer flowpath boundary through the fan rotor and closely circumscribes the tips of the fan blades in order to minimize leakage past the fan blades. Prior art fan cases are typically lined with a sacrificial abradable material in order to protect the fan blades during contact between the fan blades and the fan case (referred to as “rub.”) While sacrificial wearing away of the abradable prevents damage to costly fan blades, it also opens up the radial clearance at the blade tips, resulting in loss of engine thrust.

Some engines are subject to unavoidable fan blade rubs in operation, for example during aircraft maneuvers or transient fan unbalance. In these cases the use of an abradable material alone could result in unacceptable thrust loss.

Accordingly, there is a need for a fan casing that tolerates fan blade rubs while maintaining intended clearances.

BRIEF DESCRIPTION OF THE INVENTION

This need is addressed by the present invention, which provides a fan casing incorporating a frangible, rub-tolerant bumper.

According to one aspect of the invention, a case apparatus for a gas turbine engine includes an annular case having an interior surface with annular recess formed therein; and an annular bumper disposed in the recess, the bumper comprising a frangible material and having a low-friction contact surface, wherein the bumper is configured to permit elastic deflection in response to applied forces below a predetermined threshold.

According to another aspect of the invention, a fan apparatus includes: an annular fan case having an interior surface with annular recess formed therein; an annular bumper disposed in the recess, the bumper comprising a frangible material and having a low-friction contact surface; and a rotor carrying an array of blades mounted for rotation within the case such that the bumper is in axial alignment with tips of the blades, wherein the bumper is configured to permit elastic radial deflection in response to contact between the blades and the bumper generating applied forces below a predetermined threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIG. 1 is a schematic half-sectional view of a fan section of a gas turbine engine incorporating a fan case constructed according to an aspect of the present invention; and

FIG. 2 is an enlarged view of a portion of a fan case shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows a portion of an exemplary fan section 10 of a turbofan gas turbine engine used for powering an aircraft in flight. The fan section 10 includes a fan 12 which is rotated about a central longitudinal axis “A” by a conventional fan shaft 14 powered by a conventional low pressure turbine (not shown). The fan 12 includes a rotor disk 16 from which extends radially outwardly an array of airfoil-shaped fan blades 18 (only one shown in FIG. 1). The rotor disk 16 and the fan blades 18 may be separable from each other or they may be part of an integrally-bladed rotor or “blisk.” Each fan blade 18 has a leading edge 20, a trailing edge 22, a root 24, and a tip 26. Disposed downstream of the fan 12 is an array of airfoil-shaped outlet guide vanes (“OGVs”) 27. While the invention is described in the context of a fan and fan case, it will be understood that the principles of the present invention are equally applicable to casings surrounding other types of rotating components.

An annular fan case 28 surrounds the fan 12. As used herein, the term “annular” refers to a structure with a closed perimeter that is generally ring-shaped and includes both circular and non-circular shapes. The fan case 28 has forward and aft ends 30 and 32. A forward flange 34 mates with a nacelle (not shown) and an aft flange 36 mates with a flange 38 of a downstream engine casing component 40. The fan case 28 has an outer surface 42 and an opposed interior surface 44. The interior surface 44 cooperates with other components, described in more detail below, to define a flowpath surface “F” configured to closely surround the tip 26 of the fan blades 18.

In accordance with known practices, the fan case 28 is sized and shaped so as to be able to withstand expected operating loads such as gas pressure loads, body loads, and maneuvering loads. The fan case 28 is also configured to serve as a containment member, or in other words to resist penetration if it should be struck by a fan blade 18 released from the rotor disk 16. A blade release would typically be the result of a foreign object being ingested by the fan 12 during engine operation and is commonly referred to as a “blade-out” event. In the illustrated example, the fan case 28 is of monolithic construction and is made from an alloy such as aluminum, titanium, or steel. Fan cases may be made from composite materials as well. As used herein, the term “axial alignment” implies a common or overlapping position of two components as measured along the central longitudinal axis A.

As best seen in FIG. 2, an annular recess 46 is formed in a portion of the interior surface 44 and a bumper 48 is disposed in the recess. The bumper 48 may be an annular component having a contact surface 50 positioned in axial alignment with the fan blades 18. As discussed in more detail below, the bumper 48 is configured so as to have low friction at the contact surface 50, to permit elastic radial deflection with linear stress-strain behavior during blade contact under relatively low loads, and to be frangible during blade contact under relatively high loads. As used herein, the term “low friction” is relative and refers to a generally hard, smooth condition without a rough finish. In the illustrated example, the bumper 48 has a “hat section” shape including a generally axially-aligned web 52, a pair of spaced-apart, radially-extending legs 54, and optionally a pair of flanges 56 extending axially forward and aft from the distal ends of the legs 54. The flanges 56 are shaped to fit against the interior of the recess 46. The web 52 of the bumper 48 defines the contact surface 50 and is slightly crowned or convex-curved relative to the fan blades 18. The bumper 48 is sized and shaped such that it does not require any change in the recess 46 or the fan case 28 as compared to a prior art fan case design. It use therefore has no significant effect on the containment function of the fan case 28.

The bumper 48 is constructed so as to be frangible during a blade impact. As used herein, the term “frangible” refers to a material that will essentially disintegrate into very small, low-mass particles upon failure, i.e. it will experience brittle failure rather than ductile failure. In the illustrated example the bumper 48 is made from a composite system, for example intermediate-modulus graphite fibers in a toughened epoxy matrix. The radially-inboard surface of the bumper 48 may incorporate a glass fiber layer to minimize damage to the blade tips 26 in case of contact. The bumper 48 may be secured in position in the recess 46 with a known adhesive.

An optional filler 58 is disposed in the annular channel defined by the shape of the bumper 48. The purpose of the filler 58 is to permit control of the bumper's harmonics by providing stiffness and/or damping to the bumper 48. Materials such as composite honeycomb (e.g. incorporating an aramid fiber such as NOMEX) or elastomers may be used for this purpose. The filler 58 may be provided as one piece or as multiple pieces, and may be bonded to the bumper 48 and/or the recess 46 using a known adhesive.

An abradable material 60 of a known type may be disposed in the recess 46 in the spaces forward and aft of the bumper 48. In the illustrated example the abradable material 60 comprises a phenolic resin embedded with glass microspheres. The exposed surface of the abradable material 60 cooperates with the bumper 48 and the interior surface 44 of the fan case 28 to define the flowpath surface F. During manufacture, some or all of the interior surface 44, the abradable material 60, and the bumper 48 may be machined in one or more process steps to form the contours of the flowpath surface F.

In operation, there normally will be no contact between the bumper 48 and the fan blades 18. Occasionally the fan 12 may experience a minor unbalanced condition causing it to deflect radially from a nominal position (i.e. to whirl or gyrate instead of purely rotating). As a result the tips 36 of the fan blades 18 may deflect radially and contact the bumper 48, specifically the web 52. The contact surface 50 of the web 52 is low-friction, allowing the fan blade 18 to skate or skid along its surface, while the legs 54 permit linear elastic deflection of the bumper 48 in the radial direction when the applied forces are below a predetermined threshold. The bumper 48 may thus be considered a “rub tolerant” structure. The action of the bumper 48 prevents the fan blade tip 36 from contacting the abradable material 60. This is desirable as contact with the abradable material 60 causes the radial clearance between the flowpath surface F and the fan blades 18 to open up, and can lead to unacceptable loss of engine thrust.

In contrast, if a fan blade out event occurs, which is an event generating significantly higher forces than a blade rub, the bumper 48 will act as a frangible fuse. Upon blade contact the bumper 48 will essentially be crushed or disintegrate into very small, low-mass particles. This avoids affecting the fan blade out loads or causing secondary damage. As an example, the load at which the bumper 48 will fuse may be on the order of 40-50% greater than the load expected during occasional rubs. The characteristics of the bumper 48 may be adjusted for a particular application by selection of the dimensions of the web 52 and legs 54 as well as the type of composite material, and the number, size, and orientation of plies.

Testing of a fan case constructed as described above has demonstrated the bumper's ability to provide needed linear elastic behavior under rub loads while crushing under higher loads as required so as to not affect fan blade out loads. Testing has also demonstrated that the bumper 48 prevents thrust loss after reacting rub loads without crush or delamination.

The foregoing has described a fan case with a rub tolerant bumper. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims. 

What is claimed is:
 1. A case apparatus for a gas turbine engine, comprising: an annular case having an interior surface with annular recess formed therein; and an annular bumper disposed in the recess, the bumper comprising a frangible material and having a low-friction contact surface, wherein the bumper is configured to permit elastic deflection in response to applied forces below a predetermined threshold.
 2. The apparatus of claim 1 wherein the bumper includes a web defining the contact surface, and a pair of spaced-apart parallel legs extending from the web, wherein the web and the legs cooperatively define a channel.
 3. The apparatus of claim 2 wherein the web is convex-curved.
 4. The apparatus of claim 2 wherein the bumper includes a pair of flanges extending in opposite directions from distal ends of the legs.
 5. The apparatus of claim 1 wherein the bumper comprises carbon-epoxy composite.
 6. The apparatus of claim 5 wherein the composite includes at least one layer of glass fiber at the contact surface.
 7. The apparatus of claim 2 further comprising a filler disposed in the channel of the bumper.
 8. The apparatus of claim 7 wherein the filler comprises a honeycomb structure.
 9. The apparatus of claim 1 further comprising an abradable material disposed in the recess adjacent the bumper, such that the abradable material, the contact surface, and the interior surface cooperatively define a flowpath surface.
 10. The apparatus of claim 9 wherein the abradable material comprises a phenolic resin.
 11. A fan apparatus for a gas turbine engine comprising: an annular fan case having an interior surface with annular recess formed therein; and an annular bumper disposed in the recess, the bumper comprising a frangible material and having a low-friction contact surface; and a rotor carrying an array of blades mounted for rotation within the case such that the bumper is in axial alignment with tips of the blades, wherein the bumper is configured to permit elastic radial deflection in response to contact between the blades and the bumper generating applied forces below a predetermined threshold.
 12. The apparatus of claim 11 wherein the bumper includes a web defining the contact surface, and a pair of spaced-apart parallel legs extending from the web, wherein the web and the legs cooperatively define a channel.
 13. The apparatus of claim 12 wherein the web is convex-curved.
 14. The apparatus of claim 12 wherein the bumper includes a pair of flanges extending in opposite directions from distal ends of the legs.
 15. The apparatus of claim 11 wherein the bumper comprises carbon-epoxy composite.
 16. The apparatus of claim 15 wherein the composite includes at least one layer of glass fiber.
 17. The apparatus of claim 12 further comprising a filler disposed in the channel of the bumper.
 18. The apparatus of claim 17 wherein the filler comprises a honeycomb structure.
 19. The apparatus of claim 11 further comprising an abradable material disposed in the recess adjacent the bumper, such that the abradable material, the contact surface, and the interior surface cooperatively define a flowpath surface.
 20. The apparatus of claim 19 wherein the abradable material comprises a phenolic resin. 